Field of the Invention
The present invention relates to methods for performing a random access in a machine communication terminal or a machine communication device, and more particularly, to methods for efficiently select a preamble and performing a random access when a machine communication terminal or a machine communication device in a cellular based Internet of Things (IoT) environment continuously fails to perform a random access process.
Related Art
A cellular based IoT system for implementing an Internet of Things environment, aims at providing reliable connection based on wide coverage in a licensed band in order to provide services to massive terminals, for example, machine communication terminals or machine communication devices.
As an example of the cellular based IoT system, narrow band-IoT (NB-IoT) provides an IoT service suitable for a sensor-centered application at low-speed data rate by using a bandwidth smaller than a bandwidth of 1.4 MHz of machine type communication (MTC), that is, a bandwidth smaller than the bandwidth 1.4 MHz in both uplink and downlink, for example, 180 kHz.
In the NB-IoT, OFDMA is used in the downlink and a very small subcarrier spacing, for example, a 15-kHz subcarrier spacing may be used.
In the NB-IoT uplink, SC-FDMA may be used. In the NB-IoT uplink, a 3.75-kHz subcarrier spacing or the 15-kHz subcarrier spacing may be used.
A random access as a procedure for a kind of data transmission in which the terminal transmits data to a base station at a predetermined time for connection to the base station and data transmission by the terminal serves to start communication which starts in all terminals. In particular, a method is requested, in which the base station may successfully receive a signal during the random access process, which is transmitted from a terminal which is very far away, in which coverage of 20 dB or more is extended in order to reflect an NB-IoT service characteristic requiring a wide coverage area and may also successfully transmit a response signal to the received signal to the corresponding terminal in a long distance.
When SC-FDMA based uplink of the NB-IoT is designed, a design of a physical random access channel (PRACH) needs to be modified in order to perform an LTE random access procedure due to the reduced bandwidth of the NB-IoT and the modification may partially influence the LTE random access procedure.
The LTE random access is used for multiple purposes such as an initial access and a scheduling request at the time of configuring a radio link and a primary purpose of the random access is to prepare for the data transmission by achieving uplink synchronization. The random access may become a contention-based random access or a contention-free random access.